An ink jet printer produces images on a receiver by ejecting ink droplets onto the receiver in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
In this regard, "continuous" ink jet printers utilize electrostatic charging tunnels that are placed close to the point where ink droplets are being ejected in the form of a stream. The selected ones of the droplets are electrically charged by the charging tunnels. The charged droplets are deflected downstream by the presence of deflector plates that have a predetermined electric potential difference between them. A gutter may be used to intercept the charged droplets, while the uncharged droplets are free to strike the recording medium.
In the case of "on demand" ink jet printers, at every orifice a pressurization actuator is used to produce the ink jet droplet. In this regard, either one of two types of actuators may be used. These two types of actuators are heat actuators and piezoelectric actuators. With respect to heat actuators, a heater placed at a convenient location heats the ink and a quantity of the ink will phase change into a gaseous steam bubble and raise the internal ink pressure sufficiently for an ink droplet to be expelled to the recording medium. With respect to piezoelectric actuators, a piezoelectric material is used, which possesses piezoelectric properties such that an electric field is produced when a mechanical stress is applied. The converse also holds true: that is, an applied electric field will produce a mechanical stress in the material. Some naturally occurring materials possessing these characteristics are quartz and tourmaline. The most commonly produced piezoelectric ceramics are lead zirconate titanate, barium titanate, lead titanate, and lead metaniobate.
A continuing problem with ink jet printers is the accumulation of ink on ink jet nozzle plates, particularly around the orifice from which ink drops are ejected. The result of ink drops accumulating on the nozzle plate surface around the orifice is that it becomes wettable, causing ink drops to be misdirected, which degrades the quality of the printed image. To limit or prevent the spreading of ink from the orifice to the nozzle plate, it is common practice to coat the ink jet nozzle plate with an anti-wetting layer. Examples of anti-wetting layers are coatings of hydrophobic polymer materials such as Teflon.RTM. OR and polyimide-siloxane, or a monomolecular layer (self-assembled monolayer) of a material that chemically binds to the nozzle plate.
An ink jet nozzle plate can also be contaminated by ink drops that land on it. These "satellite" ink drops are created as a by-product of the separation process of the primary ink drop that is used in printing. Another source of contamination is caused when the primary ink drop impacts the recording material and splashes back to the nozzle plate. Where the whole nozzle plate surface has been treated with a non-wetting layer, such additional ink drops will bead-up for easy removal.
Ink drops accumulating on nozzle plates can also potentially attract dirt such as paper fibers which causes the nozzles to become blocked. Partially or completely blocked nozzles can lead to missing or misdirected drops on the recording material, either of which degrades the quality of the print.
In order to solve this problem, the nozzle plates have to be periodically cleaned. This cleaning is commonly accomplished by brushing, wiping, spraying, vacuum suction, and/or spitting of ink through the orifices. A wet wiping technique utilizing inks and ink solvents used to dilute inks can be used. Even with the presence of hydrophobic non-wetting surfaces, inks often contain various materials which may leave an undesirable residue on the ink jet printhead nozzle plate. Thus, while wiping removes ink drops from the nozzle plate, the hydrophobic non-wetting coating may be severely contaminated by ink residue. Such resulting ink-fouled coatings may subsequently be unable to effectively prevent the spreading of ink from the orifices. In addition, some mechanical cleaning processes often damage the coatings, thus causing permanent printing failure of printhead operation.
U.S. Pat. Nos. 4,643,948; 5,136,310; and 5,598,193 relate to using self-assembled monolayers of alkyl thiols, alkyl trichlorosilanes and partially fluorinated alkyl silanes on nozzle plates for an ink jet printhead. However, there is a problem with these coatings in that they have a short life and they are often found to be easily fouled by ink.
It is an object of this invention to provide a method for replenishing a coating a nozzle plate for an ink jet printhead. It is another object of this invention to provide a method for replenishing a coating a nozzle plate for an ink jet printhead wherein an anti-wetting agent is bound to a nozzle plate by electrostatic attraction.